It is well known to use diversity reception techniques in order to reduce the effects of fading. Space-diversity and polarization diversity techniques are known.
In FIG. 1 a typical antenna arrangement for providing space diversity is shown to comprise a tower 1 with triangular cross section. At each side of the tower and in the top thereof two spaced apart antennas 2 are mounted. Signals received at one of these are fed along a branch A to a first non-shown receiver and signals received at the other of these are fed along a branch B to a second non-shown receiver. The received radio signal in branch A is compared with the one received in branch B and the strongest one is selected or are the signals received in the branches combined in the base band. In a typical mobile radio system the two antennas are spaced at least 10 wavelengths meters apart.
In FIG. 1 the antenna arrangement provides sectorized cells in a cellular mobile radio system by the provision of the two antennas 2 at each side of the tower. The sectors are indicated with dashed lines and are labelled S1, S2 and S3.
In FIG. 2 a typical antenna arrangement providing polarization diversity is shown. At the top of a tower, a pole 2 or at any similar support three antennas 4 are mounted 120 degrees angularly displaced thus providing three sectors S1-S3 as shown with dashed lines. A front view of each antenna 4 is shown in FIG. 3. As appears therein each antenna comprises three vertically spaced apart antenna elements with orthogonal polarization. Each antenna element comprises two cross-laid elements 6 and 7. Elements 6 of the antennas are interconnected with schematically shown cables 8. Likewise elements 7 are interconnected with cables 9. The interconnected elements 6 together form a branch A for RF signals. The interconnected elements 7 form a branch B for RF radio signals. If a radio source, for example a mobile unit, is transmitting its RF signals will be received in branch A and branch B at the same time. Down in the radio base station the received RF signals are combined in the base band. A diversity gain is attained in environments with a high proportion of multi path signals. Because the antennas 4 do not need to be horizontally spaced apart they can be mounted under the same radome. The polarization antenna diversity arrangement requires less space than a space diversity antenna arrangement.
In the following antenna arrangements having three sectors and two branches A and B will be described, although the technology described in this application is not restricted to this. The technology described in this application may be applied to antenna arrangements using two or more sectors and two or more branches.
A typical RBS site is shown in FIG. 4. It comprises a radio base station (RBS) 10, six feeders 11 extending between the RBS and six tower mounted units (TMA) 12 each provided with a respective diversity antenna 13.
A TMA is sometimes called a mast head amplifier. It should be noted that these units need not be mounted in a tower, but may be mounted on poles, walls of buildings, building roofs etc. The same goes for the diversity antennas. The technology described in this application is therefore not restricted to amplifiers mounted in towers. A tower mounted amplifier is just a name under which a device of this kind is known to a person skilled in the art.
The antenna arrangement is similar to the one shown in FIGS. 1 and 2 and comprises three sectors S1-S3. In each sector there are two branches A and B providing diversity. The antenna arrangement is illustrated at the brackets labelled S1-S3 and A, B. Antenna 13 in sector S2 branch A would for example correspond to the cross marked antenna 2 in FIG. 1 or to branch A of antenna 4 in sector S2 in FIG. 2.
In FIG. 4 the TMAs are all identical and in the following reference is therefore only made to TMA 12 in branch A of sector S1. The antenna 13 is connected to a duplex filter 14 comprising a transmitter part (TX) 15 and a receiver part (RX) 16. An RF amplifier 17 amplifies the received filtered RX signal and supplies it to another duplex filter 18 which comprises a transmitter part (TX) 19 and a receiver part (RX) 20. The function of a duplex filter is to separate the TX signal from the RX signal allowing for amplification of the separated RX signal before it is applied to the feeder 11. Signals received on the two diversity antennas 13 of branches A and B in sector S1 are processed in a respective TMA.
The radio base station 10 comprises six duplex filters 22 each connected to a respective low noise amplifiers (LNA) 23, one for each antenna/TMA. The heart of the radio base station is the transceiver units 24, 25 (TRX1, TRX2) in which the RX signals are amplified, demodulated and diversity processed and forwarded to its destinations. Each TRX1 and TRX2 also provides TX signals which are forwarded over a respective feeder 11 to the TMA in branch A and to the TMA in branch B respectively. Each antenna 13 can transmit TX signals in the down link and can receive RX signals in the uplink.
RX signals will follow an RX chain 21R extending from the antenna 13, the RX part 16 of duplex filter 14, RF amplifier 17, the RX part 20 of duplex filter 18, feeder 11, duplex filter 22, LNA 23 to an TRX. TX signals will follow a TX chain 21T from a TRX, duplex filter 22, feeder 11, duplex filters 19 and 15, to antenna 13.
Depending on the capabilities a transceiver has and the traffic capacity a radio base station is designed for, there may be just one TRX or many more transceivers than the six shown.
The arrangement shown in FIG. 1 is called 2 way diversity with 6 feeders and three sectors.
A main drawback with the known RBS is that each TMA requires an individual feeder. For a three sector site with space or polarization diversity six feeders are required. Feeders are expensive and contribute to the costs of a site. Feeders are also relatively heavy and need to be individually clamped to the mast or tower. Each feeder will thus put a load on the tower, especially if the distance between the RBS and the TMA is long. Each feeder must also be manually clamped to the tower, a time consuming work, especially if there are many feeders that need to be clamped.
EP-A1-1100212 relates to a transmitter and receiver arrangement in which signals transmitted by four transmitters at four different frequencies are combined in hybrid combiners and broadband combiners. The four different signal frequencies are fed on a common feeder to a first terminal of a diplexer and are transmitted on a single antenna common to the four transmitters. The antenna also receives radio signals on a different RX frequency band. The diplexer provides the RX signals on a second terminal from which they are fed to a receiver on a second feeder. Accordingly there are two feeders between the diplexer and the transmitter and receiver arrangement. The TX frequencies fall within a TX frequency range which is generally non-overlapping the RX frequency range.